More refactoring of FamInst/FamInstEnv; finally fixes Trac #7524

Quite a bit of tidying up here; the fix to #7524 is actually
only a small part.

* Be fully clear that the cab_tvs in a CoAxBranch are not
  fresh.  See Note [CoAxBranch type variables] in CoAxiom.

* Use CoAxBranch to replace the ATDfeault type in Class.
  CoAxBranch is perfect here.  This change allowed me to
  delete quite a bit of boilerplate code, including the
  corresponding IfaceSynType.

* Tidy up the construction of CoAxBranches, and when FamIntBranch is
  freshened.  The latter onw happens only in FamInst.newFamInst.

* Tidy the tyvars of a CoAxBranch when we build them, done in
  FamInst.mkCoAxBranch.  See Note [Tidy axioms when we build them]
  in that module.  This is what fixes #7524.

Much niceer now.

2 files changed, 8 insertions, 6 deletions

diff --git a/compiler/vectorise/Vectorise/Generic/PAMethods.hs b/compiler/vectorise/Vectorise/Generic/PAMethods.hs
index f5cbf93434..af815c9294 100644
--- a/compiler/vectorise/Vectorise/Generic/PAMethods.hs
+++ b/compiler/vectorise/Vectorise/Generic/PAMethods.hs
@@ -23,7 +23,6 @@ import Type
 import OccName
 import Coercion
 import MkId
-import Name
 import FamInst
 
 import DynFlags
@@ -38,7 +37,8 @@ buildPReprTyCon orig_tc vect_tc repr
  = do name      <- mkLocalisedName mkPReprTyConOcc (tyConName orig_tc)
       rhs_ty    <- sumReprType repr
       prepr_tc  <- builtin preprTyCon
-      liftDs $ mkFreshenedSynInstLoc (getSrcSpan name) name tyvars prepr_tc instTys rhs_ty
+      let axiom = mkSingleCoAxiom name tyvars prepr_tc instTys rhs_ty
+      liftDs $ newFamInst SynFamilyInst False axiom
   where
     tyvars = tyConTyVars vect_tc
     instTys = [mkTyConApp vect_tc . mkTyVarTys $ tyConTyVars vect_tc]
diff --git a/compiler/vectorise/Vectorise/Generic/PData.hs b/compiler/vectorise/Vectorise/Generic/PData.hs
index cbedf8d8e0..f2b9f7ab9c 100644
--- a/compiler/vectorise/Vectorise/Generic/PData.hs
+++ b/compiler/vectorise/Vectorise/Generic/PData.hs
@@ -19,6 +19,7 @@ import BuildTyCl
 import DataCon
 import TyCon
 import Type
+import FamInst
 import FamInstEnv
 import TcMType
 import Name
@@ -45,9 +46,10 @@ buildDataFamInst name' fam_tc vect_tc rhs
  = do { axiom_name <- mkDerivedName mkInstTyCoOcc name'
 
       ; (_, tyvars') <- liftDs $ tcInstSkolTyVarsLoc (getSrcSpan name') tyvars
-      ; let fam_inst = mkDataFamInst axiom_name tyvars' fam_tc pat_tys rep_tc
-            ax       = famInstAxiom fam_inst
-            pat_tys  = [mkTyConApp vect_tc (mkTyVarTys tyvars')]
+      ; let ax       = mkSingleCoAxiom axiom_name tyvars' fam_tc pat_tys rep_ty
+            tys'     = mkTyVarTys tyvars'
+            rep_ty   = mkTyConApp rep_tc tys'
+            pat_tys  = [mkTyConApp vect_tc tys']
             rep_tc   = buildAlgTyCon name'
                            tyvars'
                            Nothing
@@ -57,7 +59,7 @@ buildDataFamInst name' fam_tc vect_tc rhs
                            False       -- Not promotable
                            False       -- not GADT syntax
                            (FamInstTyCon ax fam_tc pat_tys)
-      ; return fam_inst }
+      ; liftDs $ newFamInst (DataFamilyInst rep_tc) False ax }
  where
     tyvars    = tyConTyVars vect_tc
     rec_flag  = boolToRecFlag (isRecursiveTyCon vect_tc)